Minding Your Minerals: Iron, Zinc, Selenium, and Chromium
Nancy Clark, MS, RDTHE PHYSICIAN AND SPORTSMEDICINE - VOL 24 - NO. 6 - JUNE 96
In the quest to eat right, don't overlook minerals—they're an important part of your diet. If you are active, you may be considering taking some of the popular supplements—iron, zinc, chromium, and selenium. But just what are these minerals?
Minerals originate from the soil. Unlike vitamins, they cannot be made by people, animals, or other living systems. Minerals in the soil are taken up by plants and then get passed on to humans and other animals who eat those plants.
Minerals are often referred to as trace elements because humans need only small amounts of them. There are specific recommended dietary allowances (RDAs) for iron and zinc. But because less is known about chromium and selenium, there are only "safe and adequate" daily ranges for them. The low end of the range prevents deficiencies that can impair your body's function; the high end of the range is below toxic (poisonous) doses.
Some athletes believe that they need high doses of minerals to counter the stress of hard training. But most studies show that, except for iron (particularly among female athletes), the mineral status of highly trained athletes is similar to that of healthy, untrained people and that training does not deplete mineral status (1).
If you diet and restrict your calorie intake, however, you have a higher risk of developing minor mineral deficiencies. Low intakes of zinc and chromium can also be related to eating too many refined and processed foods and not enough whole grains. Still, a low dietary intake does not always translate into outright deficiencies. Your body tends to absorb more of a mineral when you need more.
People who take mineral supplements do so to optimize their health. But if you are taking a mineral supplement, keep in mind that nutrition professionals don't yet fully understand the effects of self-prescribing too much of too many supplements. Oversupplementing may lead to a mineral imbalance or toxicity.
The theory "if a little bit is good, more must be better" doesn't work for mineral supplements because more of one mineral may keep your body from absorbing another mineral. Absorption problems can arise when the different minerals compete for pathways through your body or when they interact with each other and form a mineral complex that is poorly absorbed.
Another danger may be taking mineral supplements in addition to eating lots of foods fortified with 100% of your RDA for iron, zinc, chromium, and other nutrients. Also, taking high doses of supplements longterm may create future health problems.
Your best bet to enhance your mineral intake is the all-natural way—eating a variety of unrefined foods, including whole grain breads and cereals, lots of fresh fruits and deeply colored vegetables, lean meats, and low-fat dairy products. And if your diet is balanced with the right amount of carbohydrates, proteins, and fats (roughly 60%, 15%, and 25% of calories, respectively), you are likely getting the right balance of minerals. But if you are deficient in a mineral, (for example, if you have anemia and need iron) your doctor will probably recommend a supplement.
Iron is a part of red blood cells and helps transport oxygen to exercising muscles. If you have low levels of iron and become anemic, your muscles will receive less oxygen and produce more lactic acid. A buildup of lactic acid results in premature fatigue when you work out. Anemia is more common among women than men because women tend to lose iron through menstrual bleeding.
The RDA for iron is 10 mg for men and 15 mg for women. The iron derived from animal foods (heme iron) is absorbed better than the iron in plant foods (nonheme iron). The best way to boost your iron intake is to eat lean meats, dried beans, wheat germ, tofu, whole grains, and fortified breakfast cereals. A side effect of iron supplements can be constipation.
Zinc is in every tissue throughout your body and is an important component in energy metabolism (using food for energy). Zinc is essential for making protein, which means it helps grow muscles and other tissues. It can also aid in injury healing and immunity against diseases. If you have a zinc deficiency, you may have rough, dry skin and you may lose your appetite and your ability to taste foods.
The RDA for zinc is 12 mg per day for women and 15 mg for men. This is the amount commonly added to multivitamin and mineral supplements. Zinc from animal meats, seafood, and eggs is far better absorbed than the zinc from plant sources. And foods high in fiber may further reduce zinc absorption. In fact, zinc from beef (no fiber) is absorbed four times more effectively than zinc from a high-fiber breakfast cereal (2).
Vegetarians tend to have lower zinc levels than do meat eaters (1). So if you don't eat meat, take extra care to choose zinc-rich plant foods, such as beans and unrefined, whole grains.
Most studies on zinc supplements show no benefits to exercise performance (3). But studies do suggest that zinc supplements between 50 mg to 100 mg might have adverse consequences, such as altered copper and iron status, impaired immune function, and a decline in "good" cholesterol (4). A good rule is to avoid zinc-only supplements because the doses can be too high.
Chromium is an essential mineral that you need in small amounts to maintain normal blood sugar balance. The reference daily intake (which is a new type of guideline similar to the RDA) for chromium is 120 micrograms per day for men and women. There is limited knowledge about the dietary needs for chromium, the amount of chromium found in foods, and the chromium status of athletes and the general public. We do know, however, that because excess chromium interacts with iron and zinc, you should keep your total intake under 200 micrograms per day (5).
If you have diabetes and are deficient in chromium, supplementation may help you control your blood sugar (5). Although chromium deficiency is not the primary cause of diabetes in the United States, if you have a family history of late-onset (type II) diabetes, you should eat chromium-rich foods such as whole grain breads, nuts, prunes, molasses, cheese, and oysters, or consider taking a daily tablespoon of brewer's yeast.
Among athletes, chromium has been touted as a way to build muscle and decrease body fat, but scientific research shows no fat-burning or muscle-building benefits (6).
Selenium is a powerful antioxidant that works together with vitamin E. Antioxidants are compounds that protect you from oxygen particles that cause cell damage that in turn can lead to cancer and other diseases. The RDA for selenium is 55 micrograms for women and 70 micrograms for men. It is unclear whether extra selenium is helpful in reducing the risk of cell damage among athletes. Because intakes greater than 200 micrograms may be toxic, the best advice to date is to limit selenium intake to the RDA.
Good food sources are meat, eggs, milk, seafood, and—depending on the amount of selenium found in soil—broccoli, garlic, mushrooms, and whole grain cereals.
The key to mineral intake is to stay in balance. The best way to do this is to eat a well-balanced diet and—if you think you may be deficient in a mineral—talk to your doctor or registered dietitian.
Remember: You, your physician, and your nutritionist need to work together to discuss nutrition concerns. The above information is not intended as a substitute for appropriate medical treatment.
Ms Clark is director of Nutrition Services at SportsMedicine Brookline in the Boston area. She is a fellow of the American College of Sports Medicine, a fellow of the American Dietetic Association, and a member of its practice group, Sports and Cardiovascular Nutritionists (SCAN).